From what I have experienced, it is related to sleep debt.
It means that if you had 5 hours of sleep but you expected to get 8, you are in debt of 3 or so hours. Those hours need to be caught up in your sleep the next day.
Another thing could be if you consume caffeine or a lot of sugar to keep your self awake. The crash that comes with it could cause you to be tired the following day.
Latest Answers